ABSTRACT Two of the major problems in attaining euglycemia in subjects with type 1 diabetes (T1D) are hypoglycemia (including fear of severe hypoglycemia) and post-meal hyperglycemia. The latter commonly occurs in adolescents secondary to missed meal and snack insulin boluses. Both of these issues will be addressed over the five years of this grant. As over half of severe hypoglycemic events occur during sleep, we will continue our development of algorithms to predict and prevent nocturnal hypoglycemia using a closed loop system (CLS) in which the information from a continuous glucose monitor (CGM) is used to suspend delivery of insulin by an insulin pump. Initial studies will be done in the Clinical Translational Research Centers (CTRCs) to document safety of the "Florence" CLS (Abbott Diabetes Care) and we will then conduct a randomized clinical trial in the home environment in years 2 and 3 of the grant. Similarly, a fully portable CLS will be used to detect and correct hyperglycemia resulting from missed food boluses. The safety and efficacy of algorithms which utilize the signal form a CGM and provide output to an insulin pump will initially be studied in the CTRC's in year 3 of the grant. In years 4 and 5 of this grant we plan to assess the combined algorithms for prevention of hypoglycemia and hyperglycemia in a fully ambulatory system developed by Abbott Diabetes Care. We believe that prevention of hypoglycemia, particularly at night, and reduction of post prandial hyperglycemia will result in improvement in glycemic control and reduction in HbA1c levels.